soc/intel/skylake: move i2c voltage config to own variable

In preparation of merging the lpss i2c config structures on
apollolake and skylake move the i2c voltage variable to its
own field. It makes refactoring things easier, and then there's
no reason for a separate SoC specific i2c config structure.

BUG=chrome-os-partner:58889

Change-Id: Ibcc3cba9bac3b5779351b673bc0cc7671d127f24
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/17347
Reviewed-by: Furquan Shaikh <furquan@google.com>
Tested-by: build bot (Jenkins)
Reviewed-by: Duncan Laurie <dlaurie@chromium.org>
diff --git a/src/mainboard/intel/kunimitsu/devicetree.cb b/src/mainboard/intel/kunimitsu/devicetree.cb
index 8dded98..3cdaa06 100644
--- a/src/mainboard/intel/kunimitsu/devicetree.cb
+++ b/src/mainboard/intel/kunimitsu/devicetree.cb
@@ -164,7 +164,7 @@
 	register "usb3_ports[2]" = "USB3_PORT_DEFAULT" # Type-A Port (card)
 	register "usb3_ports[3]" = "USB3_PORT_DEFAULT" # Type-A Port (board)
 
-	register "i2c[4].voltage" = "I2C_VOLTAGE_1V8"  # I2C4 is 1.8V
+	register "i2c_voltage[4]" = "I2C_VOLTAGE_1V8"  # I2C4 is 1.8V
 
 	# Must leave UART0 enabled or SD/eMMC will not work as PCI
 	register "SerialIoDevMode" = "{ \